Former Sec. of Defense Chuck Hagel, “The Vietnam War” co-director Lynn Novick, Sen. John McCain, “The Vietnam War” co-director Ken Burns, Anne Finucane and Brian Moynihan of Bank of America and former Sec. of State John Kerry. (Photo courtesy of Bank of America)

Before previewing scenes from his new documentary, “The Vietnam War,” at the Kennedy Center, filmmaker Ken Burns illustrated the division the unpopular war fomented in the United States decades ago with a neat audience gambit.

First, he asked those who had served to stand. A number of silver-haired men did, and the crowd erupted in applause. Then he asked those who protested the conflict to rise as well. Nearly as many did, and they were greeted with claps, too.

“I couldn’t tell the difference,” Burns said, striking a conciliatory note.

The clips he proceeded to show from the 18-hour documentary that he co-directed with Lynn Novick, which will begin airing in 10 weekly installments on PBS, underscored those not-so-long-ago tensions, along with the confusion and controversy that accompanied the bloody war. The images, many in grainy vintage film interspersed with modern-day interviews with U.S. and Vietnamese soldiers as well as others whose lives were touched, evoked the horrors of the battlefield and the anger back in the States.

But afterward, a panel that included Burns, Novick and three Vietnam veterans — former secretary of state John Kerry, former defense secretary Chuck Hagel, and Sen. John McCain (R-Ariz.) — agreed that the film might start conversations that could begin to bridge the long-standing rifts the war started, as well as offer cautionary lessons for the current moment.

(And atypically for a Washington movie screening, almost the entire audience remained seated for the post-viewing panel discussion, instead of surreptitiously scurrying for the exits — clearly, the panelists were as riveting as the onscreen drama to the crowd, which was heavy on military brass, business types, lawmakers and congressional staff.)

“It’s the right time, particularly since we are in such turmoil in the world today,” said McCain, who spent more than five years as a prisoner of war — and who was greeted at the screening with a standing ovation, a tribute to his ongoing battle with brain cancer. Among the takeaways, McCain said, were making sure military and civilian leaders are honest with the public, and avoiding a draft that relies on the lowest-income people to serve.

The Arizona Republican spoke of frequent visits to the Vietnam Veterans memorial on the Mall, a monument etched with the names of more than 58,000 dead soldiers. “These young men died because of inadequate or corrupt leadership — we must have leaders who can lead and be able to give them a path to victory so we will not sacrifice them, ever again, to a lost cause,” McCain said.

Kerry agreed, listing as the lessons applicable to today: “Knowing what we’re doing, being honest with our people, making war a last resort, exhausting diplomacy — these are all relevant to every choice we face,” he said.

And Hagel said that as painful as it might be to revisit the violent chapter, it’s instructive. “Yes, it’s difficult to see this, but it’s important … for our next set of leaders to understand the consequences of war and the consequences of decisions that we never get all right.”

“What also came out of Vietnam was the first real … questioning of our government, a questioning of our leaders and a demand for accountable government — honest leadership,” he said. “I think we’re going through a period of that right now, pretty intensely.”

And then there was another deviation from the typical script for such Washington-night events, when attendees are desperate to queue up for their Ubers and chauffeured sedans (those early-morning cable hits await!): a prolonged and heartfelt standing ovation for the panel before a performance by the Lumineers.